The present invention relates generally to a method and apparatus for monitoring a patient's heart, and more particularly to a sense amplifier for sensing and amplifying intracardiac impulses produced during contractions of the heart. The sense amplifier is particularly well adapted for use in a pacer system analyzer, wherein the operation of a pacer is monitored in association with a patient's heart prior to implantation.
To assist physicians in treating cardiac disorders of the type for which the use of implantable cardiac pacers is indicated, pacer system analyzers (PSA's) have been developed. These devices are used at the time of pacer implantation to efficiently measure the parameters of a pacer system, including a patient's heart, a pacer and implanted pacer leads, without the need to perform separate procedures requiring multiple interconnections and an undesirably long time period to complete. Pacers to be implanted are tested for proper programming and operation, not only while connected in a simulated pacing system environment, but also while operating in the actual system in which they are to be used. Moreover, pacer system analyzers are preferably equipped to generate pacing pulses as required to support the patient during the pacer implantation process, independently of the pacer to be implanted.
By using a pacer system analyzer, a physician is able to adjust the operating parameters of a pacer system as required to suit the specific needs of an individual patient before the pacer has been fully implanted and the implantation surgery completed. This minimizes the need for inconvenient and potentially injurious post-implantation adjustment of the pacer or its associated pacer leads.
In measuring several of the pacer system operating parameters, it is often necessary to sense and amplify the intracardial electrical signals which are produced in response to both naturally occurring, and artifically induced cardiac events. The sensing and amplification of these typically low-level cardiac signals is complicated not only by the presence of the periodically applied pacing pulses, but also by randomly occurring extraneous noise impulses or motion artifacts, which may exceed the intracardiac signal level by several orders of magnitude. Sense amplifier saturation, resulting from the occurrence of high level pacing pulses and artifacts, is undesirable since total amplifier insensitivity results during the period of saturation. Following saturation, amplifier gain will be reduced and will slowly increase to the normal value in a non-linear fashion as the amplifier recovers. Disruption of quiescent operating bias voltage points caused by amplifier saturation generally increases battery current drain, while increased stress on circuit components reduces reliability of the system. It is therefore desirable that sense amplifier saturation be avoided.
Prior sense amplifiers have avoided saturation by providing suitable blanking circuitry in the input or output circuits of the amplifier. The blanking circuits typically included analog switch devices which were actuated by means of a digital control signal applied during generation of each cardiac pacing pulse. Since these systems relied on the generation of appropriately timed logic control voltages, saturation could still result from random noise or from artifacts occurring between generated pacing pulses.
The present invention is directed to an unsaturable sense amplifier. To this end, the input and output circuits of the amplifier are each provided with blanking circuits which are actuated in response to input or output signals exceeding predetermined fixed limits rather than by the application of logic control voltages as in prior circuits. Accordingly, the sense amplifier as described herein will be automatically blanked by potentially saturating signals regardless of when they occur or where they originate. Thus, an essentially unsaturable sense amplifier is provided.
In view of the foregoing, it is a general object of the present invention to provide a new and improved sense amplifier for sensing intracardiac electrical signals indicative of a patient's cardiac activity.
It is a more specific object of the present invention to provide an improved sense amplifier which is not saturated by the presence of applied high level pacing pulses.
It is a still more specific object of the present invention to provide an improved sense amplifier which is not saturated by the occurrence of random noise or motion artifacts.